Saturday, March 29, 2014

Greenhouse Climate Control or: How I learned to Stop Worrying About My Plants Freezing.

Keeping plants from freezing in the greenhouse is always a concern in the winter. I've seen my greenhouse get to 100* during the day and at night it'll drop into the 30's. It just doesn't retain any heat. To deal with this I bought a thermostatically controlled outlet from Amazon. Supposedly it comes on at 35* and goes off at 45*.

I don't really trust it. I haven't been able to test it to confirm that it actually comes on at 35*. Not to mention that that is awfully close to freezing. Even if it does come on and the temperature is dropping fast, the greenhouse might not heat up in time to keep the plants alive.

The red thermostatically controlled outlet is supposed to come on at 78* and got off at 70*. I was actually outside one day and heard the fans shut off. I checked the temperature in the greenhouse and it was 67*. That's 3 degrees off from what it's supposed to be (assuming my other thermometer is correct). I can live with that for fans, not for the heat.

As a result of this I've had to manually turn the heater on at night and off in the morning. That's really inconvenient.

I found the solution in the home brewing community. Home brewers need to keep their fermentation vessels at a specific temperature and there's a nifty device called an STC-1000 that will do just that. There are a lot of forum posts, blogs, and youtube videos on how to use the thing. I went by this one.

The STC-1000 has a temperature sensor and two relays. It turns one one for heat and other to cool. It's exactly what I needed.

The first step was to cut holes in the project box. I used a Dremel to cut a hole in the lid for the receptacle and a hole in the end of the box for the STC-1000.

I broke off the tab connecting the top and bottom plugs on the receptacle. This allows them to be controlled independently. One will come on for the heat and the other to cool. I used the wiring diagram found here.

The sensor wire and power cord were run through rubber grommets. Getting the grommets in and the cords through them was probably the most challenging part of this entire build.

Here it is powered up for the first time.

I put the sensor into my chest freezer and watched the display immediately drop. I was pleased with how fast it reacted. One of the few downsides this thing has is that it only displays temperatures in Celsius. Programming the thing was a pain too. The directions that came with it were badly translated. I couldn't decipher them. Luckily Google knows everything (or at least where to find it).

I set the temperature on it to 4.4*C which is 40*F. I plugged a light into the heat outlet, put the sensor in the freezer and waited. When the panel hit the desired 4.4* the light came on.

Then I wanted to figure out how to set the temperature for the other outlet. I wanted the fans to come on at 75*F or 23.8*C. Well, it doesn't work that way. You can only set one temperature. When the sensor drops below that the heat relay comes on. If it's above that the cold relay turns on. That's not exactly what I wanted, but I can work with it.

My solution was to continue using the red thermostatically controlled outlet that I already had. I plugged it into the cold outlet. I plugged the heater and a light bulb into the hot outlet. The light will make it easy for me to know when the heat is on.

Having it come on at 40*F should give the heater plenty of time to warm up before the temperature drops further. It'll also save me electricity and money by shutting off at 40*. I won't have to leave it on all night and heat the greenhouse to 48-50*. I'm very excited about this. Of course, we're fast approaching our last frost date so it may not get used at all. Oh well, there's always next year.

Bonus tomato picture:

It's a good thing that we're getting close to our last frost date. These things have gotten huge!